The present invention relates to a method of well-to-well tomography, which is a method to obtain geologic details of an underground formation from sound waves traversing the formation. In particular well-to-well tomography is a method of obtaining a spatial model of a property of part of a subsurface formation located between underground seismic receivers.
In a way, the formation between seismic receivers can be regarded as a complex transmission medium in the sense as referred to in Applicant's International patent application publication No. 03/023450. This publication deals with a method of seismic imaging a subsurface formation, wherein a complex transmission medium is present between a set of seismic sources and a set of seismic receivers. The method described in this publication serves to reduce the adverse effects of the presence of the complex transmission medium on the seismic image of the subsurface formation. In order to enable reducing the adverse effects of the complex transmission medium, the known method proposes to create a virtual source at the position of a receiver.
To explain how the virtual source is created, we assume that there is only one seismic source (above the complex transmission medium) and one or more receivers (below the complex transmission medium). The signal reflected from the subsurface layer under the seismic receiver is adversely affected by the presence of the complex transmission medium. To remove this adverse effect, part of the signal received at the receiver (which is designated to be a virtual source) is time-reversed and convolved with the signal. The convolved signal is now a signal that is received by the receiver(s) from a virtual source at the designated receiver. This convolved signal is thus not affected by the complex transmission medium between the real source and the virtual source. Suitably, the part of the signal that is used is associated with the earlier or more direct arrivals from the source to the receiver. In addition, the virtual source will be a symmetrical wavelet at time zero.